Distillation of volatile monomeric materials from aqueous polymer dispersions while reducing foam formation by the presence of a water-soluble weak inorganic acid



I Patented Dec. 9,;1941

OLATILE MONOMERIO' MATERIALS FROM AQUEOUS POLYMER DISPERSIONS WHILE REDUCING FOAM DISTILLA'T ION F FORMATION BY THE PRESENCE OF A.

WATER-SOLUBLE WEAK INORGANIC ACID David Craig, Silver Lake, Ohio, assignor to The B. F. Goodrich Company, New York, N. Y., a corporation of New York No Drawing. Application September 23, 1943,

Serial N0. 503,535 l This invention relates to the removal and recovery by distillation of volatile materials from aqueous polymer dispersions and to a method whereby such distillations may be effected with improved efiiciency and economy. More particularly the invention relates to a method of reducing or depressing the formation of foam during the removal by steam distillation of volatile steamdistillable substances from soap-containing aque-' ous polymer dispersions. Still more particularly, the invention is directed to a method of reducing foam formation'during thesteam distillation of 9 Claims. (Cl. 202-46) monomeric styrene from synthetic rubber latices prepared by the incomplete polymerization of butadiene-l,3 and styrene in aqueous emulsion in the presence of soap.

, In the production of synthetic rubber by the emulsion polymerization of mixtures of butadi ene-1,3 and styrene, or of other mixtures of monomers comprising -a butadiene-L3 hydrocarbon and a monomer copolymerizable therewith in aqueous emulsion, the mixed monomers are emulsified in an aqueous solution of soap as an emulsifying agent; other materials including polymerization initiators and catalysts, which initiate and speed up the polymerization, and polymerization modifiers, which improve the plasticity and solubility of the product, are included in the emulsion and polymerization is then efiected by agitating the emulsion at a temperature of about 20,

to 80 C. While the polymerization may be continued until substantially all of the monomers are converted into rubbery copolymers, it is ordinarily desirable, for a number of reasons, to terminate the polymerization after from 60 to 95% of the monomers have been converted into polymer, This prevention of further polymerization is'accomplished, and at the same time oxidation and ther processing of the latex into synthetic rubher. The unpolymerized butadiene-l,3, which is quite volatile, is easilyremoved simply by subjecting the latex to vacuum, but the unpolymerized styrene, which is somewhat less volatile, is more diflicult to remove and must ordinarily be steam-distilled from the latex.

In such steam distillation, however, numerous I difficulties have arisen due to the fact that the presence of the soap renders the latex quite foamy.

Thus, theformation of foam during steam distillation not-only seriously impedes the distillation making the distillation a slow and laborious process but'also frequently becomes so great that the capacity of the equipment used is exceeded resulting in loss of material and delay in carrying out the process. Moreover, the amount of foam produced has been found to vary considerably with the precise'nature of the latex used, being dependent among other things upon the nature and concentration of emulsifying agents,

polymerization modifiers, polymerization inhibitors, and other substances used in the polymerization process, the degree to which the monomers have been converted into polymers and the particle size of the polymer in the latex. And, un-' fortunately, foaming is most pronounced when certain of the most'effective polymerization procedures are used. Furthermore, the peculiar foaming encountered, being dependent upon such a widevariety of conditions, has been found generally not to be alleviated by conventional foambreaking methods used in other more or less unrelated processes.

I have now discovered that foaming during distillation processes to remove monomeric material and other volatile substances from synthetic rubber latices and other soap-containing aqueous polymer dispersions may be substantially reduced by the inclusion of weak inorganic acids in the latex during the distillation. The weak acids have been found to act as foam depressants thereby permitting rapid and eflicient distillation of the volatile materials fromthe dispersion.

Any of the various weak inorganic acids may be employed, those acids which. are soluble in the aqueous phase of the'dispersion in the concentrations used being preferred. Weak acids are of course acids (1. e., compounds which in aqueous medium possess a pH of less than seven, which contain at least one hydrogen atom replaceable with a metallic or other positive radical to form salts or which, in terms of the ionic theory, dissociate in aqueous medium to produce hydrogen ions) which are termed weak acids" by reason of the fact that they ionize but little and yield but few hydrogen ions in aqueous medium and, therefore, possess a relatively small dissociation constant. Acids which ionize in normal solution at 18 C. only to an extent of less than 10% and,

similarly, which possess a primary dissociation constant at 25 C. of less than 1x10 but more than 1 10- (the dissociation constant of water) are generally considered to be weak acids and are so considered herein.

Weak'acids which may be employed as foam depressants in this invention include such preferred water-soluble weak inorganic acids as boric, carbonic, 'hydrosulfuric, hypochlorous and hydrocyanic acids, all of which possess a dissociation constant between 1O- and 1X as well as other weak inorganic acids (having a dissociation constant less than 1x 10-) such as nitrous, selenious, arsenic. arseniousaclds and the like.

Water-soluble inorganic acid salts which ionize in normal solution to give a hydrogen ion concentration greater than that of Water (pH greater than seven) but less than that produced by the dissociation of strong acids (dissociation constant of more than 1x10- and which, therefore, are

weak "acids, may also be employed either alone, or in connection with another weak acid which is not an acid salt thereby forming a buffer system. Examples of such acid salts which are weak acids include monosodium dihydrogen phosphate, sodium hydrogen sulfite, and the like.

' The preferred weak acid foam depressants are thewater-soluble weak inorganic acids, particularly those having a primary dissociation con stant between 5 l0- and 1 l0- and the acid salts of the type described. Specifically boric, carbonic, hydrosulfuric acids and monosodium dihydrogen phosphate and the bufier combinations of these, such as mixtures of boricacid and monosodium dihydrogen phosphate, have been found to be particularly effective The amount of the Weak acid foam-depressant to be included in the latex or other aqueous poly mer dispersion during its distillation to remove volatile substances therefrom may w be varied widely depending on the particular weak acid employed, especially its strength or dissociation constant; the nature of the dispersion being treated, particularly the dispersed materials, emulsifying agents and other substances present therein; and

the degree to which foaming is to be depressed. However, since the presence of large amounts of even weak acids in aqueous polymer dispersions containing a saponaceous emulsifying or dispersing agent, particularly the soapy dispersions ordinarily encountered, results in coagulation of the dispersion and separation of aggregates of poly- I mer, it is essential that the amount of weak acid of the latex from 0.2 to 2.0 pH units, this being done by the inclusion in the latex of about 1% by weight or less of the weak acid based on the polymer content of the latex.

The inclusion of the weak-acid foam depressant in the aqueous polymer dispersion is ordinarily most conveniently accomplished simply by adding, with stirring, the desired amount of the weak acid or an aqueous solution thereof to the dispersion just prior to carrying out the distillation process. However, other methods and procedures for accomplishing the result may also be employed. For example, if the acid to be used is an acid formed by reaction of a gas with Water such as carbonic acid, the desired amount of the gas may be bubbled into the dispersion thereby keeping the volume of the dispersion more nearly constant. Moreover, instead of adding the acid to the dispersion just prior to carrying out the distillation, it may be added during the distillation, as at the top of a distillation tower, or, in some instances, where thedispersion is prepared by emulsion polymerization, the acid may be present during the polymerization process thereby producing a non-foamy dispersion. Another alternative consists in forming the weak acid in situ by including in the dispersion a salt or ester of the acid which hydrolyzes during, the distillation to liberate the weak acid.

In the practice of the invention foaming is substantially reduced bythe use of weak acid foamdepressants in any of the various distillation processes known to the art for removing volatile substances from soap-containing aqueous polymer dispersions. The distillation process may be a batch distillation process or a continuous distillation process carried out in a suitable column or tower such as a perforated-plate or bubble-cap column; it may be carried out at any suitable pressure and temperature; and theuse of steam may or may not be employed. In the removal of monomeric styrene from synthetic rubber latices prepared by the incomplete polymerization in a soapy emulsion of a mixture of butadiene-1,3 and styrene, by steam distilling the styrene from the latex, the distillation is ordinarily accomplished by passing the latex downward through a stripping column equipped with bubble-cap or perforated plates, passing steam upward through the column and recovering a distillate comprising monomeric styrene and water at the top of the column. .A particularly useful embodiment of the invention constitutes the use of weak acid foam-depressants in such a process.

In one such embodiment unpolymerized styrene is steam-distilled without encountering foaming difficulties from a particularly foamy synthetic rubber latex as follows:

A synthetic rubber latex is prepared by polymerizing a mixture of 75 parts of butadiene-l,3 and 25 parts of styrene in an aqueous emulsion containing, in addition to the monomers, 180 parts of water, 5 parts of fatty acid soap as an emulsifying agent, 0.3 part of potassium persulfate as a polymerization inhibitor and 0.6 part of. a mixture of aliphatic mercaptans containing about 12 carbon atoms and consisting predominately of dodecyl mercaptan as a polymerization modifier, for 14 hours at a temperature of 50 C. and then adding, to stop the polymerization, 0.2 part of betanaphthol (a polymerization inhibitor) dispersed in water. In this latex approximately 78% of the monomers are converted into the synthetic'rubber copolymer while the remaining 22% of the monomers are in the monomeric or unpolymerized form.- The latex is then subjected to a vacuum to flash out the unpolymerized butadiene-1,3 or, if desired, this g of the thus obtained l atex (still containing approximately3% by volume of. monomeric styrene) is then fed at the rate of about 1.5 gallons per minute to the top of an 18" diameter Lapp stripping column equipped 'with seven porcelain bubble-cap plates and 'further equipped for steam distillation with a 7 ft. foam-separator head, a condenser. condensate-separator and means for introducing steam at the bottom of the column in a ratio of about 5 parts of steam for 1 part of styrene, in the latex. After equilibrium is established and while maintaining a latex feed of 1.5-1.75 gallons per minute, the capacity .of the column, stripping of the styrene proceeds efliciently (97% of all styrene removed) and the height of foam in the foam-separator head does not rise above about 6 ft.

When the distillation of a further quantity of the same latex except. that it has not been treated with the solution of boric acid and monosoclium dihydrogen phosphate (the foam-depressants), is attempted in the same equipment it is found that the rate of latex feed has to be maintained at only about 0.3 gallon per minute in order to keep thehe cf foam t t e permiss of 'Tablef is presented. The foaming index" listed in the table is obtained by placing a 150 gram sample of latex in a 500 ml. three necked distillation flask connected through a 200 ml. foam-head to a condenser and distillate receiver, evacuating the flask to drive off the butadiene- 1,3, introducing steam into the distillation flask and measuring the time interval between the introductionof steam and the formation of the second drop of distillate. Very foamy latices on the introduction of steam will foam to such an extent that the distillation flask and part of the foam-head are filled with a. very stable foam consequently requiring several minutes before distillation starts; conversely, in less foamy latices lesser time intervals are required for distillation to start. The latices listed in the table are all bu- -tadiene-l,3 styrene copolymer synthetic rubber concentration. in per cent of polymer content of sible limit of 7 ft., thus showing that the presthe latex are also listed.

ono 0am Foaming Latex Foam Deprdssant 2 32122 52 index,

polymer com minutes tent of latex) r i k {dfl s liiiffi jfffi ffiiiii'jj'"1:23: N011 (A) Boric acid 0.2 ,6. (A) d0 0.6 4. (A) Hydroquinone. 0. 1 5. B Latex (A) after standing 24 hrs 9. (B) 0.25 4. (Bl Boric acid 0.6 3. (B) Ethyl Carbonate (hydrolysis to carbonic acid).. 1.0 7. c M) =loroi mercaptan w (I) =bydroquinonc o) 1. a (C) Nanlroall 5 ence of the foam-depressants enables the distillation to continue at over five times the rate possible in their absence. It is also observed that distillation in the presence of the foam-depressants results in lower build-upof coagulum on the trays and bubble caps of the column.

Other similar embodiments wherein the styrene is distilled from similar synthetic rubber latices in the presence of other weak acids also show that foaming is materially reduced by the presence of weak acids in the latex. For example, it is possible to reduce the height of foam produced duringthe stripping in the column described of a somewhat less-foamy latex prepared asdescribed above except that hydroquinone is It is evident from the table that in every casethe presence of the weak acid brings about a lowering of the amount of foam produced,

the polymerization inhibitor employed, from 6.5 l

ft. to less than 3 ft. at a stripping rate of 1.5 galions per minute, by the inclusion in the latex of 0.6% based on the polymer content of the latex, of boric acid. Again the stripping rate of a latex prepared as described above except that tri -isobutyl mercaptan is employed as the polymerization modifier and hydroquinone as the polymerization inhibitor is increased from 1.2 gallons per minute topver .1.5 gallons per minute by the inclusion in the latex of 0.6%, basedon the polymer content of the latex, of boric acid, carbonic acid, hydrosulfuric acidor acetic acid.

To further illustrate the reduction in foaming brought about by the use of weak acids the data specific application to the steam-distillation of styrene from certain especially foamy butadiene- 1,3 styrene synthetic rubber latices, it is tobe understood that the invention is'by no means so limited. Thus, instead of the butadiene-1,3 styrene latices employed in the examples, the invention may be applied to the removal of styrene from other such latices prepared by emulsion polymerization using various other emulsifying agents; and, if desired, in the presence of polymerization modifiers, polymerization, initiators etc.; and, if desired, terminated by the addition of polymerization inhibitors or antioxidants. Emulsifying agents which may be used include, in addition to water-soluble fatty acid soaps such as sodium oleate, sodium stearate, sodium myristate and the like, other soapy materials such as soaps of rosin acids and the like, and mixtures of any of these. Polymerization modifiers which may be used include aliphatic mercaptans of the type used in the specific examples such as lorol" mercaptan, triisobutyl mercaptan, cetylmercaptan, terpene mercaptans, kerosene mercaptans and other mercaptans of the formula R--SH where R is an aliphatic, heterocyclic, alicyclic or aromatic radical which may contain substituent 7 groups such :as halogen, carboxy, alkoxy, nitro, carbalkoxy, thioether and the like; and other sulfur-containing organic compounds such as the sulfides and polysulfides of such mercaptans, the xanthogens, thioxanthogens and dixanthogens, thiazyl ,disulfides, thiazoles and the like. Polymerization initiators which may be used include the per-compounds such as potassium persulfate, sodium perborate, benzoyl peroxide, hydrogen peroxide, diacyl peroxides and the like and any other of the well-known polymerization initiators. Polymerization inhibitors which are added to stop the polymerization and which also often serve as antioxidants or age-resisters for the synthetic rubber, include principally aromatic amines and phenols such as hydroquinone, catechol, p-amino phenol, beta-naphthol, diphenyl amine, phenyl beta-naphthylamine, p-phenylchain conjugated dienes such as 2-chlorobutadicue-1,3, 2-methoxy butadiene-1,3, Z-cyano-butadiene-1,3*and the like either alone, in admixture with each other or with coDolymerizable compounds such as styrene, acrylonitrile, methyl acrylate, methyl methacrylate, isobutylene, methyl vinyl ketone, methyl vinyl ether, vinylidene chloride, diethyl maleate and the like, may be distilled either with steam or by vacuum distillation to remove unpolymerized volatile monomers or other volatile substances which may be prescut or to concentrate the dispersion by distilling oil a part of the water, and the method of this invention applied thereto.

Having disclosed the invention broadly it is not intended that the invention be limited except by the spirit and scope of the appended claims.

I claim: 1. In a process of steam-distilling an unpolyinerized steam-distillable monomer from an aqueous polymer dispersion prepared by the incomplete polymerization of monomeric material comprising said steam-distillable monomer in an aqueous emulsion in the presence of a soap, the method of reducing the formation of foam during the distillation which comprises including in the said dispersion prior to the distillation a watersoluble weak inorganic acid having a dissociation constant between 5X 10 and 1 10- in an amount insufficient to coagulate the dispersion.

2. In a process of steam-distilling monomeric styrene from a synthetic rubber latex prepared by the incomplete polymerization of a mixture of butadiene-1,3 and styrene-in an aqueous emulsion in the presence of soap, the method of reducing the formation of foam during the distillation which comprises. adding-to the said latex prior to the distillation a water-soluble Weak inorganic acid having a dissociation constant-between 5X10- and 1' 10 in an amount insufficient to coagulate the latex.

3. In a Process of steam-distilling monomeric I styrene from a synthetic rubber latex prepared by the incomplete polymerization of a mixture 7 of butadiene-1,3 and styrene in an aqueous emulsion in the presence of soap, the method of reducing the formation of foam during the distillation which comprises adding to the said latex prior to the distillation an aqueous solution of an inorganic acid salt having a pH less than seven in an amount insuflicient to coagulate the latex.

4. The method of claim 2 wherein the'watersoluble weak inorganic acid is boric acid.

5. The method of claim 3 wherein the aqueous solution is a solution of monosodium dihydrogen phosphate.

6. In a process of steam-distilling monomeric styrene from a synthetic rubber latex prepared by the incomplete polymerization of a mixture of butadiene-1,3 and styrene in an aqueous emulsion in the presence of soap, the method of reducing the formation of foam during the distillation which comprises adding to'the distillation boric acid and monosodium dihydrogen phosphate in a combined amount insufllcient to coagulate the latex.

7. The method of reducing the formation of foam while removing volatile steam-distillable substances from a soap-containing aqueous polymer dispersion which comprises including a water-soluble weak inorganic acid in said dispersion in an amount insuflicient to coagulate the dispersion and then steam-distilling.

8. The method of recovering monomeric styrene from a synthetic rubber latex prepared by polymerizing in an aqueous emulsion containing a soap and a polymerization modifier, a mixture of butadiene-1,3 and styrene until from 60 to of the monomers are converted into poly.-

mer and then adding a polymerization inhibitor,

which comprises adding a water-soluble weak inorganic acid to said latex in an amount insufficient to coagulate the latex, passing the latex downward through a stripping column, passing steam upward through the column and recovering a distillate comprising monomeric styrene and water at the top-of the column, the said method being characterized by reduced foam formation in the column during the distillation,

9. The method of claim 8 wherein the synthetic rubber latex is prepared by polymerizing in an aqueous emulsion in the presence of soap and a mercaptan, a mixture of butadiene-1,3 and styrene until from 60 to 95% of the monomers are converted into polymer and then adding a polymerization inhibitor selected from the class consisting of aromatic amines and phenols.

DAVID CRAIG.

REFERENCES CITED The following references are of record'in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,273,891 Pollack Feb, 24, 1942 2,240,764 Dreisback May 6, 1941 2,304,728 Boyer et al Dec. 8, 1942 FOREIGN PATENTS Number Country Date 364,089 Great Britain Dec. 21, 1931 588,785 Germany Nov. 27, 1933 OTHER REFERENCES International Critical Tables, vol. 6, pages 271- 273 (1929). (Copy in Scientific Library.)

to said latex prior 

